This invention relates to a lithium secondary battery and to a method of manufacturing a negative electrode. In particular, this invention relates to a lithium secondary battery provided with an improved negative electrode comprising a carbonaceous material, and to a method of manufacturing a negative electrode wherein the method of manufacturing the carbonaceous material is improved.
In recent years, a nonaqueous electrolyte secondary battery using lithium as a negative electrode active material has been attracting attentions as a high energy density battery. Among such nonaqueous electrolyte secondary batteries, a primary battery using manganese dioxide (MnO.sub.2), carbon fluoride [(CF.sub.2)n] or thionyl chloride (SOCl.sub.2) as a positive electrode active material has been widely used already as a power source of a timepiece or an electronic calculator, or as a backup battery of a memory.
In addition, with an increasing minimization in size and weight of various types of electronic equipment, such as VTR devices or communication equipments, a demand for a secondary battery having a high energy density as a power source for these equipments has been increased, and therefore, the development of lithium secondary battery employing lithium as a negative active material has been actively studied.
For example, studies are now continued to develop a lithium secondary battery which is featured in that the negative electrode thereof is constituted by lithium, an electrolyte is constituted by a nonaqueous electrolyte which can be prepared by dissolving a lithium salt such as LiClO.sub.4, LiBF.sub.4 or LiAsF.sub.6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL) or tetrahydrofuran (THF), or constituted by a lithium ion-conductive solid electrolyte, and a positive electrode active material is constituted by a compound which is capable of topochemically reacting with lithium such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, V.sub.6 O.sub.13 and MnO.sub.2 for instance.
However, the lithium secondary battery as mentioned above has not been put into practical use yet. This is mainly because the charge/discharge efficiency of the battery is low and the number of charge/discharge time (or cycle life) thereof is still insufficient. The cause for this poor performance is assumed to be ascribed to the fact that lithium constituting the negative electrode is degraded due to a reaction with a nonaqueous electrolyte. Namely, lithium dissolved in the nonaqueous electrolyte in the form of lithium ions during the discharging reacts with a solvent as it is precipitated at the moment of charging thereby causing the surface of the lithium to be partially inactivated. Therefore, if the charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from the collector.
For these reasons, there has been proposed to employ, as a negative electrode for a lithium secondary battery, a carbonaceous material which is capable of absorbing or desorbing lithium ions such as coke, sintered resin, carbon fibers or pyrolytic epitaxial carbon so as to prevent the degradation in performance of a negative electrode that may be brought about by a reaction between lithium and a nonaqueous electrolyte or by the precipitation of dendrite.
It is now considered that the charge/discharge of a negative electrode comprising the aforementioned carbonaceous materials is mainly performed by the movement of lithium ions entering into or getting out of an interfaces between layers constituting a laminate structure of carbon planes formed of carbon atoms (a graphite structure) in the carbonaceous material. For this reason, it is required to employ, as a negative electrode for a lithium secondary battery, a carbonaceous material which is developed more or less in graphitization. However, when a carbonaceous material to be employed as a negative electrode in a nonaqueous electrolyte is manufactured through the pulverization of a macro-crystal developed in graphitization, the decomposition of the nonaqueous electrolyte will be caused, resulting in the deterioration of capacity and charge/discharge efficiency of the battery.
Moreover, when quick charging is performed in this secondary battery provided with a negative electrode comprising a carbonaceous material or when the charging of this secondary battery is performed under conditions of low temperature of 0.degree. C. or less, the charging potential of the negative electrode may become 0V or less, inviting a precipitation of lithium metal on the carbonaceous material. As a result, the quantity of absorption and desorption of lithium ions in such a secondary battery would be decreased, thus deteriorating the discharge capacity of the secondary battery under such severe conditions.
Meanwhile, Japanese Patent Unexamined Publication H/5-251080 discloses a coin type battery provided with a negative electrode comprising a carbonaceous material containing boron, wherein the negative electrode is manufactured by sintering a mixture consisting of natural graphite and an additive selected from H.sub.3 BO.sub.3, B and B.sub.2 O.sub.3 at a temperature of 1,000.degree. C. in an Ar atmosphere for 10 hours. Further, Japanese Patent Unexamined Publication H/8-31422 discloses a lithium secondary battery provided with a negative electrode comprising a carbonaceous material containing boron, wherein the negative electrode is manufactured by a method wherein pitch coke lump obtained from coal-tar pitch is pulverized at first to obtain a powder, which is then mixed with a material selected from B, B.sub.2 O.sub.3, B.sub.4 C and H.sub.3 BO.sub.3 and graphitized. On the other hand, Tanso 1996 {No. 172} 89-94 discloses a lithium secondary battery provided with a negative electrode comprising a carbonaceous material containing boron, wherein the negative electrode is manufactured by a method wherein a mesophase pitch type carbon fiber is infusibilized at first at a temperature of 300.degree. C. in air atmosphere and then heated at a temperature of 650.degree. C. to perform a primary carbonization thereof, and after being added with B.sub.4 C, the resultant mixture is heat-treated at a temperature of 3,000.degree. C. for one hour.
However, any of these secondary batteries have failed to solve the aforementioned problems.